There are many thin film devices that include a dielectric film between conducting layers—for example, thin film batteries (TFBs) and electrochromic devices. For these devices, a pinhole in the dielectric film can compromise the function. For example, a pinhole in the dielectric film can reduce the breakdown voltage of the device, or worse still lead to a short between conducting layers and render the device useless.
FIG. 1 shows a cross-sectional representation of a typical thin film battery (TFB). The TFB device structure 100 with anode current collector 103 and cathode current collector 102 are formed on a substrate 101, followed by cathode 104, electrolyte 105 and anode 106; although the device may be fabricated with the cathode, electrolyte and anode in reverse order. Furthermore, the cathode current collector (CCC) and anode current collector (ACC) may be deposited separately. For example, the CCC may be deposited before the cathode and the ACC may be deposited after the electrolyte. The device may be covered by an encapsulation layer 107 to protect the environmentally sensitive layers from oxidizing agents. See, for example, N. J. Dudney, Materials Science and Engineering B 1 16, (2005) 245-249. Note that the component layers are not drawn to scale in the TFB device shown in FIG. 1.
In a typical TFB device structure, such as shown in FIG. 1, the electrolyte—a dielectric material such as Lithium Phosporous Oxynitride (LiPON)—is sandwiched between two electrodes—the anode and cathode. The conventional method used to deposit LiPON is physical vapor deposition (PVD) radio frequency (RF) sputtering of a Li3PO4 target in a N2 ambient. However, this deposition process can lead to a very significant yield loss due to pinholes in the UPON films, and pinhole density increases with application of increasing RF power during sputtering. One approach to minimizing pinholes involves depositing thicker films of UPON—typically one to two microns thick—and when the cathode has poor surface morphology the thickness of the LiPON may need to be greater yet. However, this is still not completely effective in removing pinholes and increases the cost of the process step due to lower throughput and more costly overhead in terms of consumed materials.
A further approach to minimizing pinholes in dielectric thin films is to increase the temperature of the substrate during deposition so as to increase the surface mobility of atoms. However, this approach does not work for materials such as LiPON, since an “amorphous” phase of LiPON is required for TFBs, and the temperatures required to substantially increase surface mobility of UPON results in undesired crystallization of the UPON. Also, this approach does not work for permeation barrier layers since temperatures high enough to increase the surface mobility of the dielectric negatively affect the polymer planarization layers.
Furthermore, there are thin film structures such as permeation barrier layers (multiple repeating layers of dielectric and planarizing polymer films) for which a pinhole in the dielectric film can compromise the function. For example, a pinhole in the dielectric film can readily lead to a hole through a permeation barrier layer.
Clearly, there is a need for deposition processes and equipment which can provide dielectric thin films with lower pinhole densities at low cost.